KR20240041265A - Steel material and mold - Google Patents

Abstract

In the present invention, 0.25 mass% ≤ C ≤ 0.37 mass%; 0.08 mass% ≤ V ≤ 0.28 mass%; 6.60 mass% ≤ Mn + Cr ≤ 7.40 mass%; Mn/Cr ≤ 0.150, Mn ≥ 0.60 mass%; Cr ≤ 6.60% by mass; Cu + Ni ≤ 0.84 mass%; 0.40 mass% ≤ Si ≤ 0.90 mass%; 0.60 mass% ≤ Mo ≤ 2.00 mass%; 0.001 mass% ≤ Al ≤ 0.080 mass%; and 0.003% by mass ≤ N ≤ 0.040% by mass, with the balance being Fe and inevitable impurities.

Description

Steel material and mold {STEEL MATERIAL AND MOLD}

The present invention relates to steel materials and molds, and more specifically to mass It relates to steel materials suitable for manufacturing large-sized molds and molds using the same.

Since stress and heat are applied repeatedly when using a mold, steel materials for molds are required to have excellent multiple properties such as hardness, impact resistance, heat check resistance, and wear resistance. Accordingly, various proposals have been made in the prior art regarding steel materials having these characteristics.

For example, Patent Document 1 discloses a hot-working tool steel containing a predetermined amount of C, Si, Mn, Cr, Mo, and V, and the balance being Fe and inevitable impurities.

Patent Document 1 states that (A) when the amount of Si is set to 0.01 mass% or more and less than 0.25 mass%, it has sufficient machinability to be industrially processed into a mold shape, and general-purpose mold steel (e.g., JIS It is possible to obtain hot-working tool steel with higher thermal conductivity than SKD61), and (B) when the amount of Mn, amount of Cr, amount of Mo, and amount of V are optimized, hardenability and impact It is disclosed that high-value hot working tool steel can be obtained.

The mold manufacturing process generally includes (a) a first step of manufacturing a steel material suitable for mold manufacturing and (b) a second step of manufacturing a mold from the obtained steel material.

The first step (manufacturing steel for a mold) includes various steps. Its main steps include melting step, refining step, casting step, homogenization heat treatment step, hot working step, normalizing step, tempering step and spheroidizing annealing step. Among these, either or both the normalizing step and the tempering step may be omitted.

The second step (manufacturing a mold from steel) includes the HT step as one of the steps.

The HT step generally includes (a) machining the nodular annealed steel into a rough mold shape (rough machining), (b) quenching the rough machined mold (H), and It includes performing tempering (T), (c) performing finish machining on the quenched and tempered mold, and (d) performing surface modification on the finished mold, if necessary.

The properties required for steel materials subject to the HT step and molds manufactured by the HT step are (1) nodular annealing (SA) properties, (2) machinability, (3) impact value when the quenching rate is low, (4) ) heat check resistance and (5) softening resistance.

To achieve high impact values even at low quenching rates, three factors must be considered: (a) small amount of coarse foreign matter, (b) fine austenite crystal grains during quenching, and (c) It is necessary to satisfy three factors for high quenchability.

However, it is not easy to manufacture steel that satisfies all of the above five characteristics. For example, SKD61, a general-purpose steel for die casting molds, is excellent in SA properties and machinability, but is inferior in impact value, heat check resistance, and softening resistance.

On the other hand, steel materials obtained by reducing the defects of SKD61 (impact value, heat check resistance, and softening resistance) are overall inferior in SA properties and machinability. That is, because the effects of alloy elements on the above five properties are contradictory to each other, it is very difficult to improve the five properties simultaneously.

Japanese Patent Publication No. 2011-001572

The purpose of the present invention is to provide a steel material that is good in all five properties of SA properties, machinability, impact value, heat check resistance, and softening resistance.

Another object of the present invention is to provide a steel material that is good in all five properties of SA properties, machinability, impact value, heat check resistance, and softening resistance even when both the mass and size of the steel material are large.

Additionally, another object of the present invention is to provide a mold made from such steel.

In order to solve the above problems, the steel according to the present invention:

0.25% by mass ≤ C ≤ 0.37% by mass;

0.08 mass% ≤ V ≤ 0.28 mass%;

6.60% by mass ≤ Mn + Cr ≤ 7.40% by mass;

Mn/Cr≤0.150;

Mn ≥ 0.60% by mass;

Cr ≤ 6.60% by mass;

Cu + Ni ≤ 0.84% by mass;

0.40 mass% ≤ Si ≤ 0.90 mass%;

0.60% by mass ≤ Mo ≤ 2.00% by mass;

0.001 mass% ≤ Al ≤ 0.080 mass%; and

0.003 mass% ≤ N ≤ 0.040 mass%

It contains, and the balance consists of Fe and inevitable impurities.

The mold according to the present invention is manufactured from steel according to the present invention and has a mass of 2000 kg or more.

The steel according to the present invention has two main characteristics. The first characteristic is that the amount of C and the amount of V are relatively small. Accordingly, it is possible to prevent a decrease in impact value due to coarse foreign substances. On the other hand, when the amounts of C and V are small, austenite grains tend to coarsen during quenching. However, by reducing the amounts of C and V, adding appropriate amounts of Al and N, and adjusting the quenching conditions, it is possible to prevent a decrease in impact value due to coarsening of austenite grains during quenching.

The second main feature is the introduction of the parameters “Mn + Cr” and “Mn/Cr” to find the optimal range of the amount of Mn and the amount of Cr, while individually defining the amount of Cr and the amount of Mn. When the amounts of Mn and Cr are optimized, SA properties are improved, a decrease in impact value due to a decrease in quenchability is prevented, and softening resistance is improved.

In particular, "quenchability" and "softening resistance", as well as "SA property" and "quenchability" are properties in which the effects of elements are opposite to each other. However, if the amount of Cr and the amount of Mn are optimized, both properties can be achieved.

In addition, “machinability” and “heat check resistance” are generally characteristics in which the effects of elements are opposed to each other. Meanwhile, in the present invention, in addition to the above two features, both machinability and heat check resistance can be achieved by optimizing the amount of Si.

Figure 1 is a diagram showing the relationship between the amount of C and the impact value.
Figure 2 is a diagram showing the relationship between the amount of V and the impact value.
Figure 3 is a diagram showing the ranges of the amount of C and the amount of V.
Figure 4 is a schematic diagram of a CCT diagram.
Figure 5 is a schematic diagram showing the difference in critical cooling rate
Figure 6 is a diagram showing the effect of the amount of Mn+Cr on the critical cooling rate.
Figure 7 is a diagram showing the effect of the amount of Mn+Cr on the impact value when the quenching speed is low.
Figure 8 is a diagram showing the effect of Mn/Cr on the hardness of a test piece after SA.
Figure 9 is a diagram showing the ranges of the amount of Mn and the amount of Cr.
Figure 10 is a diagram showing the effect of the amount of Si on machinability.
Figure 11 is a diagram showing the effect of the amount of Si on heat check resistance.
Figure 12 is a diagram showing the effect of the amount of Mo on fracture toughness.
Figure 13 is a schematic diagram illustrating the cutting position of the test piece.

Hereinafter, one embodiment of the present invention will be described in detail.

[One. steel]

[1.1. Furtherance]

[1.1.1. [Main constituent elements]

The steel material according to the present invention contains the following elements, the balance being Fe and inevitable impurities. The types of added elements, component ranges, and reasons for their limitation are as follows.

(1) 0.25 mass% ≤ C ≤ 0.37 mass%:

Fine particles (carbides, carbonitrides) with a diameter of less than 0.5 μm function as “pinning particles” that prevent the growth of austenite grains when heated for quenching. If the amount of C is too small, the amount of pinning particles will not be sufficient when heated for quenching. As a result, grains may become coarse, and steel properties such as impact value, fracture toughness value, and ductility may be deteriorated.

Additionally, when the amount of C is too small, the martensite transformation start temperature (Ms point) becomes excessively high. As a result, quenchability increases but impact value may decrease.

Additionally, if the amount of C is too small, it is difficult to obtain hardness of 45 HRC or more by tempering at 560°C to 600°C. In order to secure high heat resistance, a hardness of 45 HRC or higher is required.

Therefore, the amount of C needs to be 0.25% by mass or more. The amount of C is preferably 0.26 mass% or more, and more preferably 0.27 mass% or more.

On the other hand, if the amount of C is excessive, coarse carbides or carbonitrides may crystallize during casting. These crystallized substances become “foreign bodies” that reduce the impact value. Coarse foreign substances are difficult to dissolve and remove through heat treatment (homogenization heat treatment, normalizing, and spheroidizing annealing). Coarse foreign substances often remain without being completely dissolved even after quenching and tempering. Coarse foreign substances are dissolved and reduced in size during homogenization heat treatment, but are still observed to exceed 3 μm in diameter. Foreign matter that remains without being completely dissolved becomes the starting point of destruction and causes a decrease in impact value and fatigue strength.

Additionally, when the ingot is formed of a block-shaped or bar-shaped steel material by hot working, if the cooling rate after hot working is low, the impact value may decrease. When the amount of C is excessive, this phenomenon is clear.

Therefore, the amount of C needs to be 0.37% by mass or less. The amount of C is preferably 0.36 mass% or less, and more preferably 0.35 mass% or less.

(2) 0.08 mass% ≤ V ≤ 0.28 mass%:

V combines with C and/or N in steel to form carbides, carbonitrides and/or nitrides. All of these function as pinning particles. Therefore, if the amount of V is too small, the amount of pinning particles is not sufficient when heated for quenching.

Additionally, if the amount of V is too small, the secondary hardenability decreases during tempering. As a result, it is difficult to obtain hardness of 45 HRC or more during tempering at 560°C to 600°C.

Therefore, the amount of V needs to be 0.08 mass% or more. The amount of V is preferably 0.09 mass% or more, and more preferably 0.10 mass% or more.

On the other hand, when the amount of V is excessive, coarse foreign matter increases. Additionally, when the cooling rate after hot working is low, the phenomenon of reduced impact value may become apparent.

Therefore, the amount of V needs to be 0.28 mass% or less. The amount of V is preferably 0.27 mass% or less, and more preferably 0.26 mass% or less.

(3) 6.60 mass% ≤ Mn + Cr ≤ 7.40 mass%:

Both Mn and Cr affect quenchability. If the amount of Mn+Cr is too small, quenching properties are not sufficient. As a result, impact values can be significantly reduced, especially inside large molds (areas with low quenching rates). Therefore, the amount of Mn+Cr needs to be 6.60% by mass or more. The amount of Mn+Cr is preferably 6.65 mass% or more, and more preferably 6.70 mass% or more.

On the other hand, when the amount of Mn+Cr is excessive, thermal conductivity significantly decreases. As a result, heat check resistance deteriorates due to an increase in thermal stress. Therefore, the amount of Mn+Cr needs to be 7.40 mass% or less. The amount of Mn+Cr is preferably 7.35 mass% or less, and more preferably 7.30 mass% or less.

(4) Mn/Cr ≤ 0.150:

The ratio of the mass of Mn to the mass of Cr contained in the steel (Mn/Cr) affects SA properties. If Mn/Cr is too large, SA properties deteriorate. Therefore, to soften steel to 98 HRB or less in SA with heating temperatures exceeding the A _c3 point, the cooling rate needs to be less than 10 °C/H. As a result, the SA phase becomes longer and productivity decreases. Additionally, when the crystal grains are coarse and Mn/Cr is large, SA failure is likely to occur.

Therefore, Mn/Cr needs to be 0.150 or less. Mn/Cr is preferably 0.148 or less, and more preferably 0.145 or less.

(5) Mn ≥ 0.60 mass%:

Mn affects quenchability. If the amount of Mn is too small, quenchability deteriorates. In order to ensure quenching properties when the amount of Mn is small, it is necessary to increase the amount of Cr. However, if the amount of Cr is excessive, problems described later may become apparent.

Therefore, the amount of Mn needs to be 0.60% by mass or more. The amount of Mn is preferably 0.62 mass% or more, and more preferably 0.65 mass% or more.

(6) Cr ≤ 6.60% by mass:

When the amount of Cr is excessive, softening resistance decreases. That is, when used as a die casting mold, the surface of the mold in contact with the molten metal is heated to a high temperature, and the surface of the mold heated to a high temperature is prone to softening. If the high temperature strength decreases due to softening, the heat check resistance also deteriorates. Additionally, softening of areas with hardness exceeding the maximum hardness is significant, and adjustment of tempering hardness is difficult. This is because hardness is sensitive to changes in furnace temperature.

Additionally, when the amount of Cr is excessive, thermal conductivity decreases. As a result, thermal stress increases and heat check resistance deteriorates. Additionally, when the amount of Si is 0.50% by mass or less, machinability significantly deteriorates as the amount of Cr increases.

Therefore, the amount of Cr needs to be 6.60% by mass or less. The amount of Cr is preferably 6.55% by mass or less, and more preferably 6.50% by mass or less.

(7) Cu + Ni ≤ 0.84% by mass:

In the present invention, as described above, SA properties, quenching properties and softening resistance are secured by the balance between Cr and Mn (amount of Cr, amount of Mn, amount of Mn+Cr, ratio of Mn/Cr). On the other hand, both Cu and Ni have the effect of improving quenching properties, but deteriorate SA properties. Additionally, Cu and Ni do not significantly affect softening resistance, but rather have a significant adverse effect on SA properties. When the amount of Cu is greater than the amount of Ni, hot workability deteriorates. Therefore, the total amount of Cu and Ni is limited, and the upper limit of the total amount is limited to a range that has little effect on quenching properties and SA properties.

The “quenchability characteristic value” is used as an indicator of the influence of alloying elements on improving the quenchability of steel. The larger the value of the quenching property, the greater the effect of improving the quenching property. The quenching property value is determined depending on each type of alloy element and its added amount. The quenchability of steels with different compositions is evaluated by the added value of the hardenability characteristic value corresponding to the type and amount of alloying elements.

Here, the quenching property value when 0.10% by mass of Mn is added is 0.125. Meanwhile, the quenching property value when 0.42 mass% of Ni is added is 0.062, and the quenching property value when 0.42 mass% of Cu is added is also 0.062. That is, the quenching property value (added value) when 0.42% by mass of Cu and Ni are added each (0.84% by mass added in total) is 0.124. This value is substantially equivalent to the quenching characteristic value (= 0.125) when 0.10 mass% of Mn is added. This fact means that when the amount of Cu + Ni is 0.84 mass% or less, the effect on improving quenchability is small. When the amount of Cu + Ni is about 0.84% by mass, the influence on the increase in high temperature strength is also small.

On the other hand, when the amount of Cu+Ni is about 0.84% by mass, various problems become apparent. Specifically, for example, cracks are likely to occur during hot processing, SA properties are deteriorated, and costs increase. Therefore, the amount of Cu+Ni needs to be 0.84 mass% or less. Since the amount of Mn+Cr to ensure quenchability is 6.60% by mass or more, it is clear that when the amount of Cu+Ni is 0.84% by mass or less, it does not significantly affect quenchability. The amount of Cu+Ni is preferably 0.78 mass% or less, and more preferably 0.72 mass% or less.

(8) 0.40 mass% ≤ Si ≤ 0.90 mass%:

If the amount of Si is too small, machinability decreases, and it is difficult to perform industrially stable machining of large molds. In particular, since the steel material of the present invention is intended for manufacturing large molds, the amount of cutting is large and good machinability is required. Therefore, the amount of Si needs to be 0.40% by mass or more. The amount of Si is preferably 0.45 mass% or more, and more preferably 0.50 mass% or more.

On the other hand, when the amount of C, V, and N are large and the amount of Si is excessive, coarse crystallized materials may increase. Additionally, when the cooling rate after hot working is low, the phenomenon of lowering the impact value may become apparent. Additionally, due to a decrease in thermal conductivity, thermal stress may increase and heat check resistance may decrease when steel is used as a mold. Therefore, the amount of Si needs to be 0.90 mass% or less. The amount of Si is preferably 0.85 mass% or less, and more preferably 0.80 mass% or less.

(9) 0.60 mass% ≤ Mo ≤ 2.00 mass%:

If the amount of Mo is too small, the degree of secondary hardening during tempering decreases. Therefore, if the amount of Mo is too small, it is difficult to obtain hardness of 45 HRC or more during tempering at 560°C to 600°C. Additionally, softening resistance and high temperature strength may not be sufficient, and heat check resistance may be deteriorated. Therefore, the amount of Mo needs to be 0.60 mass% or more. The amount of Mo is preferably 0.70 mass% or more, and more preferably 0.80 mass% or more.

On the other hand, when the amount of Mo is excessive, machinability decreases. In particular, when the amount of Si is small and when the amount of Mo is excessive, machinability is significantly reduced. Additionally, if the amount of Mo is excessive, fracture toughness may decrease. This tendency is evident when the amount of Si is large. Therefore, the amount of Mo needs to be 2.00% by mass or less. The amount of Mo is preferably 1.95 mass% or less, and more preferably 1.90 mass% or less.

(10) 0.001 mass% ≤ Al ≤ 0.080 mass%:

In the steel according to the present invention, the amount of C and the amount of V are much lower than that of conventional hot work die steel (SKD61). Therefore, the amount of V-based carbides, carbonitrides, and nitrides that function as pinning particles when heated for quenching is smaller than that of SKD61. Therefore, in the present invention, AlN particles are also used to prevent the growth of austenite grains.

If the amount of Al is too small, it is difficult to reduce oxygen during refining, the amount of oxide increases, and the impact value may decrease. Additionally, when the amount of Al is too small, the amount of AlN functioning as pinning particles is not sufficient. As a result, austenite grains may coarsen when heated for quenching, and impact value, fracture toughness and/or ductility may be reduced. Therefore, the amount of Al needs to be 0.001 mass% or more. The amount of Al is preferably 0.002 mass% or more, and more preferably 0.003 mass% or more.

On the other hand, if the amount of Al is excessive, coarse alumina particles may increase, and impact value and fatigue strength may decrease. Additionally, thermal conductivity may decrease and heat check resistance may deteriorate. Therefore, the amount of Al needs to be 0.080 mass% or less. The amount of Al is preferably 0.070 mass% or less, and more preferably 0.060 mass% or less.

When Ca is added to improve machinability, the amount of Al is very important to optimize the shape of the compound.

(11) 0.003 mass% ≤ N ≤ 0.040 mass%:

In the present invention, in order to disperse AlN particles in the austenite phase during heating for quenching, the amount of N as well as the amount of Al are limited. If the amount of N is too small, the amount of AlN to function as pinning particles is not sufficient. As a result, austenite grains may coarsen during heating for quenching, and impact value, fracture toughness value, and/or ductility may decrease. Additionally, if the amount of N is too small, the amount of V-based carbonitride and nitride, which are also pinning particles, may not be sufficient. Therefore, the amount of N needs to be 0.003% by mass or more. The amount of N is preferably 0.004% by mass or more, and more preferably 0.005% by mass or more.

Meanwhile, in order to add N in an amount that exceeds the amount that can be adjusted in general refining, it is necessary to actively add N using dedicated equipment, which increases material costs. Additionally, if the amount of N is excessive, coarse crystallized materials may increase. This trend is evident when the amounts of C, Si, and V are high. Additionally, if the amount of N is excessive, the amount of coarse AlN may become too large and the impact value may decrease. Therefore, the amount of N needs to be 0.040 mass% or less. The amount of N is preferably 0.038 mass% or less, and more preferably 0.036 mass% or less.

(12) Inevitable impurities:

The steel material according to the present invention may contain inevitable impurities. Elements that may be included as impurities in the steel according to the present invention and their contents are as follows. P ≤ 0.03 mass%, S ≤ 0.006 mass%, O ≤ 0.006 mass%, W ≤ 0.30 mass%, Co ≤ 0.30 mass%, B ≤ 0.0002 mass%, Nb ≤ 0.004 mass%, Ta ≤ 0.004 mass%, Ti ≤ 0.004 mass%, Zr ≤ 0.004 mass%, Ca ≤ 0.0005 mass%, Se ≤ 0.03 mass%, Te ≤ 0.005 mass%, Bi ≤ 0.01 mass%, Pb ≤ 0.03 mass% and Mg ≤ 0.02 mass%.

In the present invention, "content" means dissolving a predetermined mass of steel (preferably 1 g or more per analysis of one element) in acid, including portions with high segregation, portions with low segregation, and portions with average segregation. It refers to the “average amount of elements in steel” obtained by derivation using chemical analysis.

[1.1.2. [sub-constituent elements]

The steel material according to the present invention may additionally contain one or two or more types of elements described later in addition to the main constituent elements and inevitable impurities described above. The types of added elements, component ranges, and reasons for their limitation are as follows.

[A. Group A]

(13) 0.30 mass% < W ≤ 2.00 mass%:

In the steel according to the present invention, the amount of C and the amount of V are smaller than that of the hot-worked die steel of the prior art, so the strength may not be sufficient depending on the application. In this case, it is effective to add W to increase strength. To obtain this effect, it is desirable that the amount of W exceeds 0.30% by mass. The amount of W is more preferably 0.80% by mass or more.

On the other hand, if the amount of W is excessive, material costs increase. Additionally, the appearance of segregation may cause deterioration of mechanical properties or an increase in anisotropy. Therefore, the amount of W is preferably 2.00% by mass or less. The amount of W is more preferably 1.50% by mass or less.

(14) 0.30 mass% < Co ≤ 1.00 mass%:

Like W, Co has the effect of increasing strength. Therefore, when the strength is insufficient, it is effective to add Co to increase the strength. To obtain this effect, it is preferable that the amount of Co exceeds 0.30% by mass. The amount of Co is more preferably 0.50% by mass or more.

On the other hand, when the amount of Co is excessive, material costs increase. Additionally, the appearance of segregation can cause deterioration of mechanical properties or increase in anisotropy. Therefore, the amount of Co is preferably 1.00% by mass or less. The amount of Co is more preferably 0.90% by mass or less.

The steel according to the present invention may contain either or both Co and W.

[B. Group B]

(15) 0.0002 mass% < B ≤ 0.0080 mass%:

When the amount of P in the steel material is relatively large, P segregated at the grain boundary reduces the grain boundary strength and the impact value decreases. Addition of B is effective for increasing grain boundary strength. In order to increase grain boundary strength, B needs to exist alone (without forming a compound) in the steel. When B forms BN, the effect of adding B disappears. Therefore, in steel materials containing N, when B is added for the purpose of increasing grain boundary strength, N needs to be bonded to elements other than B.

Specifically, N is preferably bonded to a nitride-forming element such as Ti, Zr, or Nb, which easily forms nitride. These elements are effective even at the impurity level content, but if the elements are not sufficient, it is desirable to add the elements in an amount exceeding the impurity level.

BN has the effect of improving the machinability of steel materials. Therefore, when B is added for the purpose of improving machinability, there is no need to actively add nitride forming elements to the steel material.

In order to obtain the above effect, the amount of B is preferably 0.0002% by mass or more. The amount of B is more preferably 0.0003% by mass or more, and even more preferably 0.0004% by mass or more.

On the other hand, even when B is added more than necessary, there is no difference in effect and there is no practical benefit. If the amount of B is excessive, the cost of steel increases. Therefore, the amount of B is preferably 0.0080% by mass or less. The amount of B is more preferably 0.0075 mass% or less, and even more preferably 0.0070 mass% or less.

[C. Group C]

(16) 0.006 mass% < S ≤ 0.180 mass%, (17) 0.0005 mass% < Ca ≤ 0.0500 mass%, (18) 0.03 mass% < Se ≤ 0.50 mass%, (19) 0.005 mass% < Te ≤ 0.100 mass% %, (20) 0.01 mass % < Bi ≤ 0.50 mass % and (21) 0.03 mass % < Pb ≤ 0.50 mass %:

In the steel according to the present invention, the addition of free-cutting elements is effective to improve machinability. Specific examples of free cutting elements include S, Ca, Se, Te, Bi, and Pb. The steel material according to the present invention may contain any one of these free-cutting elements, or may contain two or more.

In order to obtain sufficient free cutting properties, it is preferable that the content of each free cutting element is greater than the above lower limit.

On the other hand, if the content of the free cutting element is excessive, cracks are likely to occur during hot processing. Additionally, if the content of free cutting elements is excessive, impact value, fatigue strength, heat check resistance, etc. may be reduced. Therefore, it is preferable that the content of each free cutting element is below the above upper limit.

[D. Group D]

(22) 0.004 mass% < Nb ≤ 0.100 mass%, (23) 0.004 mass% < Ta ≤ 0.100 mass%, (24) 0.004 mass% < Ti ≤ 0.100 mass% and (25) 0.004 mass% < Zr ≤ 0.100 mass% %:

In the steel material according to the present invention, carbonitride forming elements other than V and Al may be added to increase the amount of carbides, carbonitrides and/or nitrides. Specific examples of carbonitride forming elements include Nb, Ta, Ti, and Zr. The steel material according to the present invention may contain any one of these carbonitride forming elements, or may contain two or more.

In order to prevent excessive grain growth of austenite grains, the content of the carbonitride forming element is preferably greater than the above lower limit.

On the other hand, if the content of the carbonitride forming element is excessive, the carbide, carbonitride and/or nitride are crystallized in a coarse state during casting. Coarse crystallized particles are not removed even during homogenization heat treatment, SA, and quenching and remain as foreign matter, which causes a decrease in impact value and fatigue strength. Therefore, it is preferable that the content of the carbonitride forming element is below the above upper limit.

[1.2. Characteristics of steel]

[1.2.1. mass and size]

As described above, the properties required for steel materials undergoing the HT step and molds manufactured by the HT step include five properties: SA property, machinability, impact value, heat check resistance, and softening resistance. Among these five characteristics, the problem with large-sized steel is the low impact value inside large molds manufactured with large-sized steel.

The first reason why the impact value decreases in large molds is because large foreign substances are prone to crystallization within large steel materials. This is because the solidification rate during ingot manufacturing is low in large steel materials.

The second reason why the impact value decreases in large molds is because the cooling rate after hot working is low, making it easy for carbides to precipitate.

A third reason for the reduced impact value in large molds is the reduced quenching rate inside the large steel.

In the steel material according to the present invention, the amount of C and the amount of V are small, the amount of Mn and the amount of Cr are optimized, so that the influence of large foreign matter, low cooling rate after hot working, or low quenching rate is small. That is, in the steel material according to the present invention, even when the mass and size are large, the five characteristics of SA property, machinability, impact value, heat check resistance, and softening resistance can be achieved at a high level.

For example, when the composition and manufacturing conditions of the steel are optimized, steel with a mass of 3000 kg or more can be obtained in addition to the fact that all five properties reach practical levels. If the composition and manufacturing conditions of the steel are further optimized, steel with a mass of 4000 kg or more or 5000 kg or more can be manufactured.

In addition, when the composition and manufacturing conditions of the steel material are optimized, it has the above characteristics and the minimum dimension (L _min ) among the longitudinal dimension (L ₁ ), transverse dimension (L ₂ ), and height dimension (L ₃ ) is Steels of 300 mm or more can be obtained. When the composition and manufacturing conditions of the steel are further optimized, steel with L _min of 350 mm or more or 400 mm or more can be manufactured.

Here, “longitudinal dimension (L ₁ )”, “transverse dimension (L ₂ )” and “height dimension (L ₃ )” each refer to the lengths of the three sides of the rectangular parallelepiped with the minimum volume forming the perimeter of the steel material.

[1.2.2. Hardness]

In the present invention, the “hardness of steel” is the Rockwell value obtained by (a) cutting a test piece near the center (region of low solidification rate) of the steel cross section on which SA was performed, and (b) measuring at room temperature using the test piece. (Rockwell) B scale hardness.

Figure 13 shows a schematic diagram illustrating the cutting position of the test piece. For example, if the steel material is a block material 10 of a [mm] × b [mm] × c [mm] (a ≤ b ≤ c, a ≥ 300 mm), a [mm] from the end surface in the c-axis direction Cut out the first material 12 of × b [mm] × d [mm]. The value of d is not particularly limited, but is preferably 30 mm to 70 mm.

Next, the second material 14 of e [mm] × f [mm] × d [mm] is cut out from substantially the center of the ab surface of the first material 12. The values of e and f are not particularly limited, but e = 90 mm to 120 mm and f = 130 mm to 160 mm are preferred. Additionally, a test piece for hardness measurement is cut from the second material 14 and used to measure hardness.

When SA is performed on the steel according to the present invention under appropriate conditions, the hardness of the steel is appropriately reduced, and machining becomes possible. When the composition and manufacturing conditions (especially SA conditions) of the steel are optimized, the hardness at room temperature is 98 HRB or less. When the composition and/or manufacturing conditions of the steel are further optimized, the hardness at room temperature is 97 HRB or less or 96 HRB or less.

[1.2.3. shock value]

In the present invention, the "impact value of the steel material" is (a) cutting a square bar (12 mm × 12 mm × 55 mm) near the center of the steel cross section on which SA was performed (an area with a low solidification rate), ( b) Impact value obtained by thermally refining the square rod to 44.5 HRC to 45.5 HRC by heat treatment, (c) producing an impact test piece from the tempered rod, and (d) performing an impact test at 15°C to 35°C says

The cutting position of the square bar is the same as that of the test piece for hardness measurement. That is, a square bar is cut from the second material 14 shown in FIG. 13.

The "heat treatment" for tempering the square rod is 1 cycle, (a) holding the square rod at 950°C for 1 hour, then cooling from 950°C to 750°C at 8°C/min, and then cooling from 750°C at 5°C/min. (b) cooling to 500°C, cooling from 500°C to 200°C at 0.5°C/min, cooling from 200°C to below 100°C at a random cooling rate, (b) then heating to a temperature range of 560°C to 600°C, and Cooling to 100℃ or lower refers to a treatment performed more than once.

“Impact test specimen” is in accordance with JIS Z2242:2018 (10 mm refers to a test piece.

“Impact value (J/㎠)” refers to the absorbed energy [J] divided by the cross-sectional area of the bottom of the notch of the test specimen (0.8 [㎠]).

“Average impact value (J/cm2)” refers to the average value of the impact value of 10 or more (preferably 10 to 20) impact test specimens.

“Low impact value rate (%)” is the ratio of the number (n) of impact test specimens with an impact value of less than 20 [J/cm2] to the total number (n ₀ ) of impact specimens subject to impact test (= n × 100/n ₀ ) says.

In the steel material according to the present invention, when the composition and manufacturing conditions are optimized, a steel material that exhibits high impact value despite its large size and small fluctuation in impact value can be obtained.

Specifically, when the composition and manufacturing conditions are optimized, a steel material with an average impact value of 25 [J/cm2] or more and a low impact value rate of 30% or less can be obtained.

When the composition and manufacturing conditions are further optimized, the average impact value is 26 [J/cm2] or more or 27 [J/cm2] or more.

Additionally, when the composition and manufacturing conditions are further optimized, the low impact value ratio is 20% or less or 10% or less.

[1.2.4. Prior austenite grain size]

The “prior austenite grain size” of steel can be determined by (a) producing an impact test specimen or a test specimen heat-treated under the same conditions as the impact specimen, (b) corroding the specimen to reveal old austenite grain boundaries, or by crystal orientation analysis. Determine the austenite grain boundaries, (c) observe the test specimen enlarged to include more than 50 grains in one field of view, and calculate the equivalent circle diameter of each old austenite grain included in the field of view. This refers to the value obtained.

“Average value of old austenite grain size” refers to the average value of the grain size of all old austenite grains included in one or more fields of view with a total area of 0.5 mm2 or more.

Because the steel material according to the present invention contains a relatively large amount of pinning particles, there is little possibility that austenite grains will coarsen when heated for quenching. When the composition and manufacturing conditions of the steel are optimized, the average value of the old austenite grain size is 150 ㎛ or less. When the composition and manufacturing conditions of the steel are further optimized, the average value of the old austenite grain size is 135 ㎛ or less or 120 ㎛ or less.

[2. mold]

The mold according to the invention is manufactured from the steel according to the invention and has the following characteristics.

[2.1. mass and size]

The steel material according to the present invention has excellent SA properties, machinability, impact value, heat check resistance, and softening resistance even when its mass and size are relatively large. Therefore, by using these steels, a mold with excellent impact value, heat check resistance, and softening resistance can be obtained even when the mass and size are relatively large.

If the composition and manufacturing conditions of the mold are optimized, molds with a mass of more than 2000 kg can be obtained, in addition to the fact that the impact value, heat check resistance and softening resistance reach practical levels. If the composition and manufacturing conditions of the mold are further optimized, molds with a mass of 3000 kg or more or 4000 kg or more can be manufactured.

In addition, when the composition and manufacturing conditions of the mold are optimized, it has the above characteristics and has the minimum dimension (L) among the longitudinal dimension (L' ₁ ), the transverse dimension (L' ₂ ), and the height dimension (L' ₃ ). A mold with ' _min ) of 250 mm or more can be obtained. If the composition and manufacturing conditions of the mold are further optimized, molds with L' _min of 300 mm or more or 350 mm or more can be manufactured.

Here, "longitudinal dimension (L' ₁ )", "transverse dimension (L' ₂ )" and "height dimension (L' ₃ )" refer to the lengths of the three sides of the minimum volume rectangular parallelepiped that forms the perimeter of the mold, respectively. .

[2.2.Hardness]

In the present invention, “hardness of the mold” refers to the Rockwell C scale hardness obtained by (a) cutting a test piece from the surface of the mold, and (b) performing measurement at room temperature using the test piece.

Alternatively, a portable hardness tester (durometer) can be used to evaluate the surface of the mold according to the Rockwell C scale. Additionally, hardness (e.g., Shore hardness) evaluated by a portable hardness tester (durometer) may be converted to Rockwell C scale hardness.

The mold according to the present invention can achieve high hardness even when its mass and size are relatively large. If the composition and manufacturing conditions of the mold are optimized, the hardness at room temperature is 38 HRC to 48 HRC. When the composition and manufacturing conditions of the mold are further optimized, the hardness at room temperature is 39 HRC to 49 HRC or 40 HRC to 50 HRC.

[2.3. shock value]

In the present invention, the "impact value of the mold" is the impact obtained by (a) cutting an impact test piece conforming to JIS Z2242:2018 from the center of the thickest part of the mold, and (b) performing an impact test at 15°C to 35°C. It says value.

The details of “impact value”, “average impact value”, and “low impact value rate” are the same as above, and their description is omitted.

In the mold according to the present invention, if the composition and manufacturing conditions are optimized, a mold that exhibits high impact value despite its large size and small fluctuation in impact value can be obtained.

Specifically, if the composition and manufacturing conditions are optimized, a mold with an average impact value of 25 [J/cm2] or more and a low impact value rate of 30% or less can be obtained.

When the composition and manufacturing conditions are further optimized, the average impact value is 26 [J/cm2] or more or 27 [J/cm2] or more.

Additionally, when the composition and manufacturing conditions are further optimized, the low impact value ratio is 20% or less or 10% or less.

[2.4. Old austenite grain size]

The “old austenite grain size” of a mold can be determined by (a) making a test piece from the center of the thickest part of the mold, (b) etching the test piece to expose the old austenite grain boundaries, or by crystal orientation analysis. (c) This refers to the value obtained by enlarging and observing the test piece to include more than 50 grains in one field of view, and calculating the equivalent circular diameter of each old austenite grain included in the field of view.

“Average value of old austenite grain size” refers to the average value of the grain size of all old austenite grains included in one or more fields of view with a total area of 0.5 mm2 or more.

Because the mold according to the present invention contains a relatively large amount of pinning particles, there is little possibility that austenite grains will coarsen when heated for quenching. When the composition and manufacturing conditions of the mold are optimized, the average value of the old austenite grain size is 150 ㎛ or less. When the composition and manufacturing conditions of the mold are further optimized, the average value of the old austenite grain size is 135 ㎛ or less or 120 ㎛ or less.

[3. Manufacturing method of steel]

The method for manufacturing steel according to the present invention includes (a) a first step of melting mixed raw materials to have a predetermined composition, refining the molten metal, and casting the molten metal into a mold, (b) performing homogenization heat treatment on the ingot. a second step, (c) a third step of hot working the ingot after homogenization heat treatment, (d) if necessary, a fourth step of normalizing the rough material after hot working, (e) if necessary, It includes a fifth step of tempering the molding material and (f) a sixth step of performing spheroidizing annealing on the molding material.

[3.1. Step 1]

First, the mixed raw materials to have a predetermined composition are melted, the molten metal is refined, and the molten metal is cast into a mold (first step). Melting conditions, refining conditions, and casting conditions are not particularly limited, and optimal conditions can be selected depending on the purpose.

[3.2. Step 2]

Next, homogenization heat treatment is performed on the ingot (second step). Homogenization heat treatment is performed to homogenize the components by weakening the segregation of components that occurs during solidification and dissolving as much as possible the foreign substances crystallized during solidification. The conditions for homogenization heat treatment are not particularly limited, and the optimal conditions can be selected depending on the purpose.

[3.3 Step 3]

Next, hot working is performed on the ingot after the homogenization heat treatment (third step). Hot processing is performed to turn the ingot into a molding material of the desired shape. The conditions for hot working are not particularly limited, and the optimal conditions can be selected depending on the purpose.

[3.4. Step 4]

Next, if necessary, the modeling material after hot processing is normalized (fourth step). Normalizing is performed when it is necessary to homogenize and refine the structure of the modeling material. Normalizing conditions are not particularly limited, and optimal conditions can be selected depending on the purpose. The normalizing step may be omitted.

[3.5. Step 5]

Next, the molding material is tempered as necessary (step 5). Tempering is performed when it is necessary to temper martensite or bainite generated in the cooling process after normalizing, or when it is necessary to precipitate carbides in preparation for spheroidizing annealing. Tempering conditions are not particularly limited, and the optimal conditions can be selected depending on the purpose. The tempering step may be omitted.

[3.6. Step 6]

Next, spheroidizing annealing is performed on the modeling material (sixth process). As a result, the steel material according to the present invention is obtained.

In the present invention, "spheroidization annealing (SA)" refers to (a) a temperature of [A _c3 point - 10°C] to [A _c3 point + 50°C] in a furnace (hereinafter also referred to as "SA treatment temperature") (b) heating the steel material to form a structure in which carbides are dispersed in the austenite phase and little or no ferrite phase, and (b) a slow cooling method or a constant temperature maintenance method for the steel material having the structure. refers to the processing that applies.

The "slow cooling method" is (a) controlled cooling is performed from 5 ℃/H to 60 ℃/H from the SA treatment temperature to transform the mother phase into ferrite and grow carbides, and (b) when austenite is removed, controlled cooling is performed. This refers to a method of removing steel from the furnace.

In this case, it is desirable to select optimal conditions for the cooling rate of controlled cooling depending on the composition and particle size of the steel material. The temperature at which austenite is removed is about 550°C to 800°C, depending on the composition of the steel and the cooling rate during controlled cooling.

The "constant temperature maintenance method" is (a) cooling from the SA treatment temperature at an arbitrary cooling rate and maintaining constant temperature in the temperature range of 620°C to 780°C to transform the mother phase into ferrite and grow carbide, (b) This refers to a method in which constant temperature maintenance is terminated when austenite is removed and the steel is taken out of the furnace.

In this case, the holding time while the austenite is removed is about 1 hour to 24 hours depending on the composition and particle size of the steel.

SA treatment temperatures are often between 830°C and 950°C, depending on the composition of the steel. Regardless of whether the slow cooling method or the constant temperature holding method is used, the hardness of the steel after SA is approximately 98 HRB or less (approximately 240 Hv or less), and is in a soft state that is easy to rough process (process into a rough mold by machining). there is.

[4. Mold manufacturing method]

The method of manufacturing a mold according to the present invention includes (a) a first step of roughing a nodular annealed steel material, (b) a second step of performing quenching and tempering on the rough-processed mold, (c) quenching and tempering the quenched and tempered mold. a third step of performing finishing processing on the mold; and (d) a fourth step of performing surface modification on the finished mold, if necessary.

The method and conditions of each step are not particularly limited, and the optimal method and conditions can be selected depending on the purpose.

[5. effect]

The properties required for steel materials subject to the HT step and molds manufactured by the HT step are (1) nodular annealing (SA) properties, (2) machinability, (3) impact value when the quenching rate is low, (4) ) heat check resistance and (5) softening resistance. Below, we mainly use die casting as an example to explain why these five characteristics are necessary.

[5.1. SA surname]

It is desirable to perform SA until the austenite is completely removed and then take the steel out of the furnace. When untransformed austenite remains in the steel when the steel is taken out of the furnace, the austenite is transformed into bainite or martensite by cooling after the steel is taken out of the furnace.

240 Hv 이하)가 혼합되어 있다.These steels include (a) a hard part (400 Hv or more) containing bainite or martensite and (b) a soft part (98 HRB or less) containing a part where carbides are dispersed in the matrix of ferrite, that is, SA structure.

240 Hv or less) are mixed.

The state in which the hard and soft parts are mixed is called “SA defect,” and is likely to occur in steel materials with high quenchability, or when austenite grains are coarse during SA heating.

When manufacturing a mold from a steel material with SA defects by the HT step, the following three problems occur. (A) During rough machining, the hard part severely wears the machining (cutting) tool, shortening the tool life. (B) Hard parts appear as a pattern on the surface of the rough-processed mold, and a smooth surface cannot be obtained. (C) During heating for quenching, coarse austenite grains are generated near the hard part, which lowers the impact value of the mold and shortens the life of the mold.

Therefore, the steel material for molds is required to be a steel material that does not have a "hard part" with a hardness of 400 Hv or more and can be softened to 240 Hv or less by SA treatment, that is, a "steel material with good SA properties."

However, steels with good SA properties generally have poor quenching properties. Additionally, steels with good SA properties generally have high C and low Mn in many cases. In these steels, carbides are likely to precipitate during cooling during quenching and ferrite transformation is also likely to proceed, so it is difficult to obtain a bainitic or martensite structure (quenchability is low). That is, in general, “good SA properties” and “high quenchability” are contradictory to each other.

Meanwhile, the steel material according to the present invention has optimized amounts of Cr and Mn. Therefore, the steel material according to the present invention exhibits good SA properties without significantly impairing quenching properties.

[5.2. machinability]

Steel materials subject to machining are required to not cause significant wear of processing tools even when processed at high speeds. When tool wear is severe, tool replacement frequency increases and processing costs increase. On the other hand, when the processing speed is lowered to avoid tool wear, the processing efficiency decreases. For the above reasons, steel materials for molds are required to be able to be processed efficiently at low cost, that is, to have “good machinability.”

On the other hand, steel materials with good machinability generally contain a large amount of Si, P and/or S. Molds made from these steels generally have poor heat check resistance. “Poor heat check resistance” means that heat checks are prone to occur and develop. Below, the reason will be explained.

High-Si steels have low thermal conductivity. When a mold made of steel with low thermal conductivity is used for die casting, the temperature amplitude of the mold surface increases, thereby increasing the generated thermal stress.

Molds made from high P steel have low toughness. Therefore, when such a mold is used for die casting, cracks are likely to occur and propagate.

Additionally, high-S steels contain relatively large amounts of sulfides. In molds made from such steel materials, sulfides function as the origin or propagation path of cracks, and thus cracks are prone to occurrence and propagation.

That is, steel materials containing relatively large amounts of Si, P and/or S have good machinability. However, when such steel is used as a mold, high thermal stress acts on the matrix. Therefore, heat check, which is a thermal fatigue crack, is likely to occur and progress. In other words, “good machinability” and “good heat-check resistance” are contradictory to each other.

On the other hand, the steel material according to the present invention contains substantially no P and S and has an optimized amount of Si. Therefore, the steel material according to the present invention exhibits good machinability without significantly impairing heat check resistance.

[5.3. Impact value when quenching speed is low]

The mold is used after being tempered to a predetermined hardness by quenching and tempering. Molds require not only hardness but also high impact values. This is because molds with high impact values are unlikely to crack severely.

In order to obtain a high impact value, it is necessary to satisfy the following three items. That is, (a) a small amount of coarse foreign matter, (b) fine austenite grains upon quenching, and (c) high quenchability.

[5.3.1. A small amount of coarse foreign matter]

“Foreign matter” refers to a material having a composition different from that of the matrix, such as carbide, nitride, carbonitride, sulfide, oxide, etc.

In the present invention, “coarse foreign matter” refers to a foreign matter with a size (= equivalent circle diameter) of 3 μm or more.

When stress acts on the mold, coarse foreign matter easily becomes the starting point of a crack and a propagation path for the crack that occurs. Therefore, in order to obtain a high impact value, the smaller the number of coarse foreign substances, the better.

Foreign matter includes foreign matter containing one metal element and foreign matter containing two or more metal elements. Since conventional steel for die casting molds contains a large amount of C and V, the coarse foreign matter is usually composed of V-containing carbides or carbonitrides. The size and amount of V-based carbides or carbonitrides are affected not only by the chemical composition of the steel, but also by the solidification rate during casting and the temperature and time of homogenization heat treatment.

[5.3.2. Fine austenite grains during quenching]

When martensite or bainite is created from austenite, the austenite grains are divided into several packets. Each packet is divided into a plurality of strip-shaped blocks. In this case, the shorter the martensite or bainite block, the more difficult it is for the crack to propagate. The block cannot exceed the austenite grain boundaries.

Therefore, when the austenite grains are fine during quenching, the blocks of the transformed structure are also fine, and the impact value increases. During quenching, the austenite grain size is affected not only by the chemical composition of the steel, but also by the heating temperature and holding time for quenching.

[5.3.3. High quenching ability]

As the size of the mold increases, the cooling rate during quenching decreases. This trend is especially noticeable inside molds. Therefore, as the size of the mold increases recently, the cooling rate inside the mold decreases, and a decrease in impact value becomes a problem.

[5.3.4. high impact value]

From the above situation, there is a strong demand for a steel that can obtain high impact values even when the quenching rate is low, that is, a “steel with good quenchability.” In other words, “good quenchability” means that coarse bainite is not formed even when the quenching rate is low.

On the other hand, steels with good quenching properties have poor SA properties. Generally, steels with good quenchability have low C, high Ni, and high Mn. This is because, in these steel materials, carbides are difficult to precipitate and ferrite transformation is difficult to proceed during the slow cooling of SA, making it difficult to obtain an SA structure (a structure in which carbides are dispersed in the ferrite matrix).

Meanwhile, the steel material according to the present invention has optimized amounts of Cr and Mn. Therefore, the steel material according to the present invention exhibits good quenching properties without significantly impairing SA properties.

[5.4. Heat check resistance]

The surface of a die casting mold undergoes a cycle of temperature rise due to contact with molten metal and cooling due to application of a release agent. This temperature amplitude generates thermal stress, and mechanical stress due to mold clamping or injection also acts, causing cracks (heat checks) due to fatigue on the mold surface. When molten metal enters a crack and solidifies, the solidified portion transfers to the casting surface, causing unsightly appearance.

Therefore, the mold is required to be difficult to cause heat checking, that is, to have “good heat checking resistance.”

In this way, “good machinability” and “good heat-check resistance” are contradictory to each other. Generally, steels with good heat check resistance have small amounts of Si, P, and S. However, these steels tend to adhere to cutting tools, contain a small amount of S compounds that provide a lubricating effect on the cutting surface, have high toughness, are sticky, and are inferior in machinability.

On the other hand, the steel material according to the present invention contains substantially no P and S and has an optimized amount of Si. Therefore, the steel material according to the present invention exhibits good heat check resistance without significantly impairing machinability.

[5.5. softening resistance]

The temperature of the surface of the die casting mold rises due to contact with the molten metal. When the number of casting shots increases, the cumulative time of exposure to high temperatures also increases, which may reduce the hardness of the mold surface. This softening causes a decrease in high-temperature strength, and as a result, heat check resistance deteriorates.

For the above reasons, die casting molds are required to be difficult to soften, that is, to have “high softening resistance.” However, it is important to note that steels with improved softening resistance by reducing the Cr content have low high-temperature strength. This is because low Cr steel has poor solid solution strengthening at high temperatures. A decrease in high temperature strength deteriorates heat check resistance. In other words, “good softening resistance” and “good heat-check resistance” are contradictory to each other.

Meanwhile, the steel material according to the present invention has optimized amounts of Cr and Mn. Therefore, the steel material according to the present invention exhibits good softening resistance without significantly impairing quenchability and heat check resistance.

Example

[One. Appropriate element amount verification test]

[1.1. outline]

Hereinafter, the items to be achieved in the present invention will be described again.

(1) SA-type

(2) machinability

(3) Impact value when quenching speed is low

(a) A small amount of coarse foreign matter

(b) Fine austenite grains upon quenching.

(c) High quenchability

(4) Heat check resistance

(5) softening resistance

In the following verification test, items other than (3)(a) were targeted. There are three reasons:

The first reason is that (3)(a) can only be accurately verified with steel manufactured from industrial-sized ingots (mass of 8 tons or more) with a low solidification rate.

The second reason is that when manufacturing large steel materials using ingots weighing more than 8 tons, the cost and inspection time are excessive.

The third reason is that because the influence of (3)(a) is very large, in order to accurately verify the influence of (3)(b) or (3)(c) on the shock value, the influence of (3)(a) This is because it must be excluded.

Therefore, a steel material with a small cross section (diameter: 82 mm × length: approximately 3000 mm) was manufactured from an ingot with a high solidification rate (a small ingot with a mass of 150 kg). Next, heat treatment simulating an industrial manufacturing method (i.e., a method of manufacturing large mold steel and large molds) was performed on the test piece manufactured from the steel material. In this way, it is believed that the properties of the steel "other than (3)(a)" can be appropriately evaluated when manufacturing molds from ingots of industrial size.

Meanwhile, (3)(a) was evaluated as a steel manufactured from an ingot of actual industrial size and a low solidification rate (an ingot with a mass of 8 tons or more).

[1.2. [Verification test of upper limit of C quantity]

[1.2.1. Sample production]

[A. Production of round bars in SA state]

Hereinafter, the decrease in impact value when the amount of C exceeded 0.37 mass% was verified.

The composition (mass%) of the steel was set to 0.90 Si-0.06 Cu-0.13 Ni-0.81 Mn-6.23 Cr-1.79 Mo-0.019 Al-0.028 N-0.28 V, and the amount of C was systematically changed. These steel grades were cast into 150 kg ingots. After ingot production, homogenization heat treatment, hot processing, normalizing, tempering, and SA were performed. In this verification, normalizing and tempering were performed, but these treatments can be omitted. Through the above steps, a steel material (round bar) in SA state with a diameter of approximately 82 mm × length of approximately 3000 mm was manufactured.

[B. [Heat treatment of square bars simulating hot working]

Ten square bars measuring 12 mm × 12 mm × 55 mm were manufactured from round bars in SA state.

The obtained square bars were heated to reproduce the austenite grain size during industrial hot processing. That is, each rod was maintained at 1240°C in vacuum for 2 hours. At the industrial stage, heat treatment is not necessarily performed in vacuum, but since the purpose of verification is to simulate the temperature history, heat treatment was performed in vacuum.

Next, heat treatment simulating cooling after hot working the ingot into a large-sized steel material was performed on the square bar. That is, after maintaining at 1240°C for 2 hours, each rod was cooled at 1°C/min to 1000°C, and then at 0.5°C/min from 1000°C to 600°C.

In the case of steels with a large amount of C, carbides were precipitated at the austenite grain boundaries in this temperature range. The carbide had a rod-shaped, V-shaped, W-shaped, or wavy shape, and had a size of 0.5 μm to 3 μm in the long axis direction. The precipitated carbide was smaller in size than the foreign matter crystallized during solidification in the large steel, but was intermittently distributed at the grain boundaries to cover the grain boundaries. When such grain boundary carbides exist, fracture is likely to occur at grain boundaries, and the impact value is significantly reduced.

In the temperature range below 600°C, an inert gas was introduced into the vacuum furnace and pressurized to 3 Torr to 4 Torr (0.40 kPa to 0.53 kPa), and the inert gas was further forced to convect to rapidly cool the rod.

Cooling rates below 600°C do not simulate large-sized industrially manufactured steels. However, since the purpose of this evaluation is to investigate “the influence of grain boundary carbides precipitated in the high temperature range after hot working,” the purpose can be achieved even when the cooling history below 600°C is the above-mentioned history. The grain boundary carbides are not removed even through “normalizing-tempering-SA-quenching and tempering” after hot processing, and ultimately remain in the mold, greatly reducing the impact value.

[C. Normalizing, tempering, SA, quenching and tempering of each bar]

Next, normalizing, tempering, and SA were performed in vacuum according to an industrial manufacturing method on the square bar that had undergone heat treatment simulating hot working.

Additionally, each rod in the SA state was vacuum quenched. That is, each rod was maintained at 950°C in vacuum for 1 hour. Next, the inert gas was introduced into the vacuum furnace and pressurized to 3 Torr to 4 Torr (0.40 kPa to 0.53 kPa), and the inert gas was further forced to convect to rapidly cool the rod to 100°C or lower.

The cooling time from 950°C to 100°C during quenching was within 60 minutes. In other words, cooling of square bars is different from cooling of industrially manufactured large-sized steel materials. However, since the purpose of this evaluation is to investigate “the influence of grain boundary carbides precipitated in the high temperature range after hot working,” the purpose can be achieved even when quenching is quenching.

Subsequently, the square bar after quenching was additionally tempered. Tempering was performed by maintaining the rod at 560°C for 2 hours and then cooling it to 100°C or lower.

Additionally, tempering was additionally performed. That is, the rod was maintained at 560°C to 600°C for a predetermined time and then cooled to 100°C or lower. This treatment was performed more than once, and each bar was tempered to 44.5 HRC to 45.5 HRC. The holding temperature, time, and number of treatments were changed depending on the steel type (amount of C). This is because the softening resistance changes as the amount of C changes.

[1.2.2. Test Methods]

Impact test specimens were produced from square bars tempered to 44.5 HRC to 45.5 HRC. The impact test specimen had a shape according to JIS Z2242:2018 (10 mm × 10 mm × 50 mm, arc radius of the notch tip: 1 mm, notch depth: 2 mm, cross-sectional area below the notch bottom of the test specimen: 0.8 cm2). Impact tests were performed at 15°C to 35°C using the obtained impact test specimens.

Shock value was used for evaluation. The impact value [J/㎠] mentioned here is the absorbed energy [J] divided by the cross-sectional area of 0.8㎠ at the bottom of the notch bottom of the test specimen, and refers to the average value of 10 test specimens.

[1.2.3. result]

The impact value required for a die casting mold is 20 J/cm2 or more for a mold with a small load, and 25 J/cm2 or more for a mold with a large load. In die casting molds with an impact value of 30 J/cm2 or more, the risk of fracture is significantly reduced.

In this evaluation, quenching is rapid cooling, but when quenching is actually slow cooling in a large mold, the impact value decreases by about 5 J/cm2. Therefore, here, the threshold value of the impact value for performing quality determination was set to 25 J/cm2.

Figure 1 shows the relationship between the amount of C and the impact value. Unlike large molds, for the impact specimens used in testing, quenching is rapid cooling. Nevertheless, in cases where the amount of C was excessive, the test specimen had an impact value of less than 20 J/cm2. From this, it can be seen that the influence of grain boundary carbides precipitated in the high temperature range after hot processing is very significant. From Figure 1, it can be seen that steel grades with a C content of 0.37% by mass or less have an impact value of 25 J/cm2 or more.

[1.3. [Verification test of upper limit of V quantity]

[1.3.1. Production and impact testing of impact test specimens]

Hereinafter, the decrease in impact value when the amount of V exceeded 0.28% by mass was verified.

The composition (mass%) of the steel was set to 0.37 C-0.90 Si-0.04 Cu-0.13 Ni-0.82 Mn-6.21 Cr-1.97 Mo-0.013 Al-0.028 N, and the amount of V was systematically changed. These steel grades were cast into 150 kg ingots. Afterwards, in the same manner as the verification test for amount C, an impact test piece was produced and an impact test was performed.

[1.3.2. Result of impact test]

Figure 2 shows the relationship between the amount of V and the impact value. Unlike large molds, for the impact specimens used in testing, quenching is rapid cooling. Nevertheless, when the amount of V was excessive, the test specimen had an impact value of less than 20 J/cm2. From this, it can be seen that the influence of grain boundary carbides precipitated in the high temperature range after hot processing is very significant. From Figure 2, it can be seen that steel grades with a V amount of 0.28% by mass or less have an impact value of 25 J/cm2 or more.

[1.3.3. Appropriate range of C and V amounts]

Figure 3 shows the range of amounts of C and amounts of V. In the present invention, the amount of C and the amount of V were limited in consideration of “hardness,” “amount of coarse foreign matter,” “amount of intergranular carbides precipitated after hot processing,” and “amount of pinning particles.” Conventional hot-working die steel has a C content of 0.32 mass% or more and a V content of 0.30 mass% or more. Meanwhile, in the steel according to the present invention, the amount of C is 0.37 mass% or less and the amount of V is 0.28 mass% or less, so it can be seen that the ranges of the amount of C and the amount of V are different from those of conventional steel.

Additionally, the present invention differs from conventional steels in its quenching temperature. Quenching temperatures for conventional steels are as high as 1010°C to 1040°C to sufficiently dissolve C and V. Meanwhile, the quenching temperature of the present invention with small amounts of C and V can be as low as 920°C to 980°C, which is advantageous in that the deformation of the quenched mold is small and cracks are unlikely to occur in the mold.

[1.4. [Verification test of lower limit of Mn + Cr amount]

[1.4.1. Overview of critical cooling rate]

Hereinafter, it was verified that quenching properties were improved by optimizing the amount of Mn+Cr, and therefore, high impact values could be obtained even when the quenching rate was low.

CCT diagrams are often used to evaluate quenchability. The transformation point was determined based on the dimensional change (contraction -> expansion -> contraction) during the step of cooling the steel at a constant rate from the quenching temperature.

Figure 4 shows a schematic diagram of a CCT diagram. In FIG. 4, “●” represents the transformation temperature determined according to the above criteria, and represents data at which a certain cooling rate differs from the quenching temperature. ● Based on this, draw a line corresponding to the transformation temperature. Solid lines represent highly accurate lines, and dashed lines represent estimated lines. In Figure 4, the cooling rate across the boundary between martensite and bainite (line of arrows) is the critical cooling rate X.

Critical cooling rate X is greatly influenced by Mn, Cr, etc. Figure 5 is a schematic diagram of the difference in critical cooling rate In Figure 5, the position of bainite in steel B is to the right (long time side) of that in steel A. This indicates that X of steel B is smaller than X of steel A.

In other words, steel B tends to have a fine structure (martensite, or bainite transformed at low temperature) even when X is small. Likewise, steels with small X are considered to have “good quenchability.” Steels with good quenchability are easy to quench even in “large molds” with small X and tend to have a fine structure.

[1.4.2. Production of round bars in SA state]

Below, the effect of the amount of Mn+Cr on quenchability and impact value was verified.

The composition (mass%) of the steel was set to 0.32 C-0.45 Si-0.08 Cu-0.11 Ni-1.06 Mo-0.18 V-0.028 Al-0.011 N, and the amounts of Mn and Cr were systematically changed. These steel grades were cast into 150 kg ingots. Afterwards, in the same manner as the verification test for quantity C, a steel material (round bar) in SA state with a diameter of approximately 82 mm × length of approximately 3000 mm was manufactured.

[1.4.3. Test Methods]

[A. Measurement of critical cooling rate]

A test piece having a substantial shape of 4 mm in diameter x 10 mm in length was produced from a round bar in the SA state. The test specimen was heated to a quenching temperature of 950°C and cooled to below 100°C at a random constant rate. The transformation point was determined based on the dimensional change at that time (contraction -> expansion -> contraction). Additionally, the critical cooling rate X was determined based on the obtained transformation point.

[B. Impact test]

Ten square bars of 12 mm × 12 mm × 55 mm were produced from the round bar in SA state. Each rod was maintained at 950°C in vacuum for 1 hour and then quenched. The cooling rate during quenching was 8℃/min from 950℃ to 750℃, 5℃/min from 750℃ to 500℃, 0.5℃/min from 500℃ to 200℃, and cooling rate from 200℃ to below 100℃. was not specifically controlled.

The quenching step is one of the examples assuming the interior with the lowest cooling rate when quenching a large mold weighing 2000 kg or more. Since the phase transformation is substantially complete when the temperature reaches 200°C, the cooling rate from 200°C to 100°C or lower was not specifically controlled.

Subsequently, each bar was tempered. Tempering was performed by maintaining the rod at 560°C for 2 hours and then cooling it to 100°C or lower.

Additionally, tempering was additionally performed. That is, the rod was maintained at 560°C to 600°C for a predetermined time and then cooled to 100°C or lower. This treatment was performed more than once, and each bar was tempered to 44.5 HRC to 45.5 HRC. The holding temperature, time, and number of treatments were changed depending on the steel type (amount of Mn and amount of Cr). This is because the softening resistance changes when the amount of Mn and/or the amount of Cr changes.

Impact test specimens were produced from square bars tempered to 44.5 HRC to 45.5 HRC. The impact test specimen had a shape according to JIS Z2242:2018 (10 mm × 10 mm × 50 mm, arc radius of the notch tip: 1 mm, notch depth: 2 mm, cross-sectional area below the notch bottom of the test specimen: 0.8 cm2). Impact tests were performed at 15°C to 35°C using the obtained impact test specimens.

Shock value was used for evaluation. The impact value [J/㎠] mentioned here is the absorbed energy [J] divided by the cross-sectional area of 0.8㎠ at the bottom of the notch bottom of the test specimen, and refers to the average value of 10 test specimens.

[1.4.4. result]

[A. critical cooling rate]

Figure 6 shows the effect of the amount of Mn+Cr on the critical cooling rate. From Figure 6, it can be seen that as the amount of Mn+Cr increases, the critical cooling rate decreases, that is, the quenchability increases.

[B. Impact test]

From the above, it was found that as the amount of Mn+Cr increases, the quenching property increases. Hereinafter, the effect of increasing the amount of Mn+Cr was verified based on the impact value when the quenching rate was actually low. This is because even if quenchability is high (even if X is small), the impact value does not increase when the martensite transformation point is high.

Figure 7 shows the effect of the amount of Mn+Cr on the impact value when the quenching rate is low. From Figure 7, it can be seen that the impact value increases as the amount of Mn+Cr increases. In Figure 7, the cooling rate was set to 0.5°C/min in the temperature range of 500°C to 200°C where transformation of martensite or bainite occurs.

Steel grades with an amount of Mn+Cr of 6.60% by mass or more had an impact value of 25 J/cm2 or more. In the case of a steel grade in which the amount of Mn+Cr was 6.60% by mass, one test piece out of ten had an impact value of less than 20 J/cm2.

In addition, steel grades with an Mn+Cr amount of 6.70% by mass or more had an impact value of 30 J/cm2 or more. In the case of a steel grade with an amount of Mn+Cr of 6.70% by mass, none of the 10 test specimens had an impact value of less than 20 J/cm2.

No coarse foreign matter was observed in the fracture cross section of the impact test specimen. There is no guarantee that foreign matter does not exist throughout the steel. However, in the evaluation of 10 impact test pieces, the number of test pieces with an impact value of less than 20 J/cm2 was 1 or less, so even if foreign matter exists, the amount is estimated to be very small.

After the impact test, the destroyed test piece was corroded and the metal structure was observed. As a result, the average value of the old austenite grain size was as fine as 25 μm to 80 μm.

From the above, the effects of (3)(b) and (3)(c) could be confirmed under the condition that the effect of (3)(a) was very small.

[1.5. Verification test of upper limit of Mn/Cr]

[1.5.1. Production of test pieces]

Below, the effect of Mn/Cr on SA properties was verified.

In the same manner as in the positive verification test of Mn+Cr, round bars in the SA state were manufactured. Next, a test piece (12 mm × 12 mm × 20 mm) for evaluation of SA properties was produced from the round bar in SA state. The test piece was subjected to vacuum heat treatment simulating "hot working-normalizing-tempering-spheroidizing annealing" in the manufacturing process of large mold materials, and SA properties were evaluated. In this verification, normalizing and tempering were performed, but these treatments can be omitted.

The details of the vacuum heat treatment step are as follows. That is, first, the test piece was maintained at 1240°C in vacuum for 0.5 hours. This treatment is a treatment that reproduces the coarse grains of hot-worked materials. After maintaining the temperature at 1240°C for 0.5 hours, nitrogen gas was introduced into the furnace, the nitrogen gas was pressurized to cause forced convection, and the test piece was cooled to 100°C or lower.

The test piece was then maintained at 970°C in vacuum for 1 hour. This processing simulates normalizing. After maintaining at 970°C for 1 hour, nitrogen gas was introduced into the furnace, the nitrogen gas was pressurized to cause forced convection, and the test piece was cooled to 100°C or lower.

The test piece was then maintained at 680°C in vacuum for 6 hours. This treatment simulates tempering. After maintaining the temperature at 680°C for 6 hours, nitrogen gas was introduced into the furnace, the nitrogen gas was pressurized to cause forced convection, and the test piece was cooled to 100°C or lower.

Finally, SA was performed. That is, the test piece was maintained at 900°C in vacuum for 1 hour. Thereafter, the test piece was cooled to 600°C at 15°C/H in vacuum. After that, nitrogen gas was introduced into the furnace, the nitrogen gas was pressurized to cause forced convection, and the test piece was cooled to 100°C or lower.

Although the vacuum heat treatment step is not exactly the same as the step of manufacturing large-sized mold steel, important conditions for evaluating SA properties, such as SA heating temperature and cooling rate, can simulate industrial steps.

[1.5.2. Evaluation of hardness]

After SA, the hardness of the test specimen was evaluated at room temperature. Figure 8 shows the effect of Mn/Cr on the hardness of the test specimen after SA. From Figure 8, it can be seen that the hardness after SA is 98 HRB or less when Mn/Cr is set to 0.150 or less. Additionally, from Figure 8, it can be seen that the hardness after SA is 97 HRB or less when Mn/Cr is set to 0.145 or less.

Additionally, when Mn/Cr was 0.145 or less, even if the cooling rate after SA (cooling rate from 890°C to 600°C) was increased to 20°C/H, the hardness after SA softened to 98 HRB or less.

[1.5.3. Appropriate range of amount of Mn and amount of Cr]

From the above, the appropriate ranges of the amount of Mn and the amount of Cr were determined. Figure 9 shows the ranges of the amount of Mn and the amount of Cr. In Figure 9, the pentagonal area surrounded by five lines is the range of the steel material according to the present invention.

In the present invention, by introducing parameters such as the amount of Mn, the amount of Cr, the amount of Mn+Cr, and Mn/Cr, Mn in which (1) SA property, (3) quenching property, and (5) softening resistance are maintained high. A narrow range of amounts of and Cr was found. In addition, it was possible to achieve both (1) SA property and (3) quenching property with opposing effects of elements, and both (3) quenching property and (5) softening resistance with opposing effects of elements.

[1.6. Verification test of lower and upper limits of Si amount]

[1.6.1. Production of round bars in SA state]

Hereinafter, the effect of the amount of Si on machinability and heat check resistance was verified.

The composition (mass%) of the steel was set to 0.31 C-0.81 Mn-0.08 Cu-0.11 Ni-6.19 Cr-1.01 Mo-0.18 V-0.028 Al-0.011 N, and the amount of Si was systematically changed. These steel grades were cast into 150 kg ingots. Afterwards, in the same manner as the verification test for quantity C, a steel material (round bar) in SA state with a diameter of approximately 82 mm × length of approximately 3000 mm was manufactured. All steel grades were softened below 98 HRB.

[1.6.2. Test Methods]

[A. Evaluation of quenching properties]

A test piece measuring 53 mm × 53 mm × 200 mm was produced from a round bar in SA state. The test piece was cut with a cutting tool, and the amount of wear of the cutting tool was measured. The cutting distance at which the wear amount of the cutting tool reached 300 μm was defined as the tool life. It was determined that the longer the cutting distance, the better the quenching properties.

[B. Heat resistance test]

A cylindrical test piece with a diameter of 72 mm and a thickness of 50 mm was produced from a round bar in the SA state. Next, the cylindrical test piece was quenched and tempered in the same manner as the impact test piece used in Figure 7 (verification test of the amount of Mn+Cr) and tempered to about 45 HRC.

A temperature cycle of heating and cooling was applied to one end face (72 mm diameter face) of the tempered test piece. High-frequency heating to 580°C and cooling to below 100°C with jetted water were set to 1 cycle, and 10,000 cycles were applied.

The test piece after 10,000 cycles was cut vertically near its center, and the depth of the crack (heat check) generated on the heated and cooled side was evaluated. Heat check resistance was evaluated by selecting three cracks in descending order of depth among the plurality of cracks observed and averaging the depths of the three cracks (hereinafter also referred to as "crack depth").

[1.6.3. result]

[A. machinability test]

Figure 10 shows the effect of the amount of Si on machinability. When the amount of Si was less than 0.40% by mass, the tool life was equivalent to that of conventional steel, which was evaluated as having “poor machinability.” When the amount of Si is less than 0.40% by mass, machinability is poor and the number of steps in the cutting operation is enormous. In particular, since the cutting amount is large in large molds, it is difficult to achieve industrial processing if machinability is poor.

From Figure 10, it can be seen that when the amount of Si is 0.40 mass% or more, the tool life is 15 m or more. Additionally, it can be seen that when the amount of Si is 0.80% by mass or more, a tool life equivalent to SKD61, which is evaluated as having “very good machinability,” can be obtained.

[B. Heat resistance test]

Figure 11 shows the effect of the amount of Si on heat check resistance. When the amount of Si exceeded 0.9% by mass, the crack depth was equivalent to that of existing steel (SKD61, etc.) with poor heat check resistance. In large molds, in addition to the fact that it is not easy to repair the heat check, the load acting on the mold is also large, so it is easy for large cracks to occur starting from the heat check. From Figure 11, it can be seen that the amount of Si is preferably 0.90% by mass or less.

[1.7. Verification test of lower and upper limits of Mo amount]

[1.7.1. Production of round bars in SA state]

Hereinafter, the effect of the amount of Mo on fracture toughness was verified.

The composition (mass%) of the steel was set to 0.28 C-0.48 Si-0.82 Mn-0.07 Cu-0.12 Ni-6.23 Cr-0.18 V-0.026 Al-0.010 N, and the amount of Mo was systematically changed. These steel grades were cast into 150 kg ingots. Afterwards, in the same manner as the verification test for quantity C, a steel material (round bar) in SA state with a diameter of approximately 82 mm × length of approximately 3000 mm was manufactured. All steel grades were softened below 98 HRB.

[1.7.2. Measurement of fracture toughness]

A plate material of 13 mm × 62 mm × 65 mm was manufactured from a round bar in SA state. Next, the sheet was quenched and tempered in the same manner as the impact test specimen (verification test of the amount of Mn+Cr) used in Figure 7, and tempered to about 45 HRC.

Fracture toughness test specimens (test specimens with a notch and two holes) with a substantial shape of 12.5 mm × 61 mm × 64 mm were fabricated from the sheet. Additionally, fracture toughness was evaluated at room temperature.

[1.7.3. result]

Figure 12 shows the effect of Mo on fracture toughness. In both cases where the amount of Mo was too small and when the amount of Mo was too large, the fracture toughness value decreased. To prevent rapid growth of cracks in the mold, the higher the fracture toughness, the better. From Figure 12, it can be seen that when the amount of Mo is 0.60 mass% or more and 2.00 mass% or less, the fracture toughness value is 30 MPa·m ^0.5 or more.

[2. [Verification test using large ingot]

[2.1. outline]

In the verification test for the appropriate element content, steel with a small cross-section was manufactured using a small (150 kg) ingot for research purposes, and the industrial manufacturing method (steel for large molds and manufacturing method for large molds) was simulated on test pieces made from steel materials. A heat treatment was performed. Therefore, it was possible to appropriately evaluate the properties “other than 3(a)” when the steel was manufactured by an industrial manufacturing method and made into a mold.

Meanwhile, in the following examples, the effect of the present invention was confirmed by actually using an ingot with a mass of 8 tons or more. In this case, the steel was quenched and tempered, and the internal impact value was verified. That is, “3(a)” above was verified. This is because the verification of other characteristics has been completed in the verification test for the appropriate element amount.

[2.2. Sample production]

[2.2.1. Production of block materials]

Table 1 shows the composition of steel (Examples 1 to 13 and Comparative Examples 1 to 3) whose properties were verified. Comparative Example 1 corresponds to JIS SKD6 (AISI H11). Comparative Example 2 is equivalent to a commercially available steel with the Si-Mn-Cr ratio of SKD6 adjusted, and is a steel with superior quenching properties and heat check resistance than SKD6. Comparative Example 3 is a steel in which the amount of C and the amount of V exceed the upper limit of the present invention. Although not listed in Table 1, each steel contains impurity elements such as P in a range that does not exceed the above upper limit.

These steels were cast into ingots with a mass of approximately 21 tons. A 21-ton ingot has a lower solidification rate than an ingot of about 10 tons, so coarse foreign matter is more likely to affect the impact value. Under such unfavorable conditions, the validity of the amount of C and the amount of V was verified.

After homogenizing heat treatment at a high temperature for a long time on a 21-ton ingot, it was formed into a block shape by hot processing. Normalizing, tempering, and spheroidizing annealing were performed on this block material to finally obtain a spheroidized annealing material measuring 740 mm × 1060 mm × 2440 mm (mass of approximately 15 tons). The difference in mass between the ingot and the block material (approximately 6 tons) is the mass of the part removed due to quality or shape issues.

Appropriate conditions for normalizing, tempering, and spheroidizing annealing were set depending on the steel type. For example, the heating temperature for normalizing and spheroidizing annealing was set to a temperature above the A _c3 point and a temperature at which undissolved carbides exist. Additionally, the tempering temperature was set lower than the A _c1 point. The transformation point and amount of undissolved carbides vary depending on the steel type.

[2.2.2. Preparation of second material]

15000 kg)의 c축 방향 단부로부터 제1 재료(12)를 잘라내었다. 다음으로, 제1 재료(12)의 ab면의 실질적인 중심부에서 제2 재료(14)를 잘라내었다. 본 실험에서는 d = 35 mm, e = 95 mm 및 f = 135 mm가 설정되었다.The area with a lot of coarse foreign matter is near the center where the solidification rate is low. Therefore, as shown in Figure 13, the block material 10 (a = 740 mm, b = 1060 mm, c = 2440 mm, w

The first material 12 was cut from the c-axis direction end of 15000 kg). Next, the second material 14 was cut from substantially the center of the ab surface of the first material 12. In this experiment, d = 35 mm, e = 95 mm, and f = 135 mm were set.

[2.3. Test Methods]

[2.3.1. Hardness]

A small piece of 15 mm × 15 mm × 35 mm was cut from the edge of the second material 14 (95 mm × 135 mm × 35 mm). The small pieces were ground and polished, and adjusted to have parallelism and surface roughness that allow hardness measurement. Using this small piece, the Rockwell B scale hardness was measured at room temperature.

[2.3.2. Impact test]

Twenty square bars of 12 mm × 12 mm × 55 mm were produced from the second material 14 (95 mm × 135 mm × 35 mm). The obtained rods were quenched.

In Examples 1 to 13 and Comparative Example 3, each rod was maintained at 950° C. in a vacuum for 1 hour and then quenched. The cooling rate during quenching was 8℃/min from 950℃ to 750℃, 5℃/min from 750℃ to 500℃, and 0.5℃/min from 500℃ to 200℃, and cooling from 200℃ to below 100℃. Speed was not specifically controlled.

In Comparative Examples 1 and 2, each rod was maintained at 1030° C. in a vacuum for 1 hour and then quenched. The cooling rate during quenching was 8°C/min from 1030°C to 750°C, and the cooling rate below 750°C was the same as in Examples 1 to 13.

The quenching step is one of the examples assuming the interior with the lowest cooling rate when quenching a large mold weighing 2000 kg or more. Since the phase transformation is substantially complete when the temperature reaches 200°C, the cooling rate from 200°C to 100°C or lower was not specifically controlled.

Subsequently, tempering was performed by maintaining the rod at 560°C for 2 hours and then cooling it to 100°C or lower. In addition, additional tempering was performed on the square bars by maintaining them at 560°C to 600°C and then cooling them to 100°C or lower. This additional tempering was performed more than once, and each bar was tempered to 44.5 HRC to 45.5 HRC. The temperature, time, and number of treatments for maintenance were changed depending on the steel type. This is because softening resistance changes when the content of alloy elements changes.

Twenty impact test specimens were produced from 20 square bars tempered to approximately 45 HRC. Additionally, impact tests were performed at 15°C to 35°C.

[2.3.3. Old austenite grain size]

After the impact test, the test specimen was corroded to expose the old austenite grain boundaries. In addition, the metal structure was observed under a microscope and the old austenite grain size was measured.

[2.4. result]

[2.4.1. Hardness]

In both Examples 1 to 13 and Comparative Examples 1 to 3, the hardness after SA was 98 HRB or less. In the case of Mn/Cr ≤ 0.150, it was confirmed again that the steel was sufficiently softened by nodular annealing.

[2.4.2. Impact test]

Table 2 shows the average impact value, the number of test specimens with an impact value of less than 20 [J/cm2], and the low impact value rate. In all of Examples 1 to 13, the average impact value was 25 [J/cm2] or more, and the low impact value rate was 30% or less. In Examples 1 to 13, in addition to high quenchability (large amount of Mn+Cr), the amount of C and V are small, so the amount of coarse foreign matter is small. As a result, impact values were stable at high levels, even with slow quenching of material cut near the center of a large-cross-section material with a low solidification rate. However, because coarse foreign matter derived from C or V was not completely absent, and foreign matter not containing C or V was also present, test specimens with an impact value of less than 20 [J/cm2] were produced with a low probability.

On the other hand, in Comparative Examples 1 to 3, the average impact value was less than 25 [J/cm2], and the low impact value rate exceeded 30%. In Comparative Examples 1 to 3, in addition to low quenchability (small amount of Mn+Cr), the amount of C and V were large, so the amount of coarse foreign matter increased. As a result, the impact value was reduced when slow quenching was performed on material cut from near the center of large cross-section material with a low solidification rate.

Comparative Example 3 is a steel in which the amount of C and the amount of V were increased compared to Examples 1 to 13. Therefore, in Comparative Example 3, although quenching properties were high, the amount of coarse foreign matter increased. As a result, the impact value of Comparative Example 3 was smaller than that of Examples 1 to 13. That is, in the case of large cross-section materials, it is clear that the impact value cannot be evaluated based on quenchability alone, and the importance of reducing the amount of C and V can be confirmed.

[2.4.3. Old austenite grain size]

In Examples 1 to 13 and Comparative Example 3, the average value of the old austenite grain size was 30 μm to 120 μm. Meanwhile, in Comparative Examples 1 and 2, the average value of the old austenite grain size was 25㎛ to 75㎛. That is, none of the steel types had a coarse grain structure (a structure with an average grain size exceeding 150 μm) that adversely affected the impact value.

In Examples 1 to 13, the amounts of C and V were small, so the crystal grains were likely to become coarse. However, by optimizing the amount of Al and N, and quenching temperature, coarsening of crystal grains could be prevented. This is thought to contribute to stabilizing the impact values of Examples 1 to 13 at a high level. However, the average value of the prior austenite grain size in Examples 1 to 13 tended to be larger than that of the sample used in the evaluation of FIG. 7 (sample prepared from a 150 kg ingot).

[3. Versatility]

As described above, the verification was conducted assuming a die casting mold, but the present invention can be applied to molds and parts used in various types of casting as well as die casting. In addition to casting, the present invention can also be applied to molds and parts used for forging, hot stamping, extrusion, injection molding of resin, blow molding of resin, molding or processing of rubber or fiber-reinforced plastic, etc.

In the above verification, the steel was quenched from 950°C, tempered at 560°C to 600°C, and tempered to about 45 HRC to evaluate the properties. However, the steel was adjusted to a wide range of hardness at a wide range of quenching and tempering temperatures depending on the application. Steel can be applied to molds and parts.

In the verification of properties, molten block material was used, but the steel material according to the present invention can also be used as powder, bar material, wire material, or plate material.

For example, when using the steel according to the present invention as a powder, the present invention can be used in various sequential forming methods such as additive manufacturing (selective laser melting (SLM) method, laser metal deposition (LMD) method, etc.) and plasma powder welding (PPW). ) can be applied.

When the steel according to the present invention is used as a molten bar, a mold or part can be manufactured from the steel. Alternatively, when the steel according to the present invention is used as a molten bar or wire, the steel can be applied to welded additive manufacturing or repair using TIG welding or laser welding.

When using the steel material according to the present invention as a plate material, a mold or part can be manufactured by joining the steel materials. Of course, a mold or part can also be manufactured by joining members made of steel according to the present invention.

As described above, the steel material according to the present invention can be applied to members having various shapes. It is possible to manufacture or repair molds or parts using various methods from materials having various shapes made of steel according to the present invention.

Although the embodiment of the present invention has been described in detail as above, the present invention is not limited to the above embodiment, and various modifications may be made without departing from the gist of the present invention.

This application is based on Japanese Patent Application No. 2022-151231 filed on September 22, 2022, the contents of which are incorporated herein by reference.

The steel material according to the present invention can be used as a mold or its parts for various processing such as casting, forging, hot stamping, extrusion, injection molding, and blow molding.

Claims (4)

0.25% by mass ≤ C ≤ 0.37% by mass;
0.08 mass% ≤ V ≤ 0.28 mass%;
6.60% by mass ≤ Mn + Cr ≤ 7.40% by mass;
Mn/Cr≤0.150;
Mn ≥ 0.60% by mass;
Cr ≤ 6.60% by mass;
Cu + Ni ≤ 0.84% by mass;
0.40 mass% ≤ Si ≤ 0.90 mass%;
0.60% by mass ≤ Mo ≤ 2.00% by mass;
0.001 mass% ≤ Al ≤ 0.080 mass%; and
0.003 mass% ≤ N ≤ 0.040 mass%
A steel material containing, and the remainder consisting of Fe and inevitable impurities. According to paragraph 1,
Additionally, a steel comprising at least one group selected from the group consisting of group A, group B, group C and group D.
Here, group A is at least one element selected from the group consisting of 0.30 mass% < W ≤ 2.00 mass% and 0.30 mass% < Co ≤ 1.00 mass%,
Group B is 0.0002 mass% < B ≤ 0.0080 mass%,
Group C is 0.006 mass% < S ≤ 0.180 mass%, 0.0005 mass% < Ca ≤ 0.0500 mass%, 0.03 mass% < Se ≤ 0.50 mass%, 0.005 mass% < Te ≤ 0.100 mass%, 0.01 mass% < Bi ≤ It is at least one element selected from the group consisting of 0.50 mass% and 0.03 mass% < Pb ≤ 0.50 mass%,
Group D is at least selected from the group consisting of 0.004 mass% < Nb ≤ 0.100 mass%, 0.004 mass% < Ta ≤ 0.100 mass%, 0.004 mass% < Ti ≤ 0.100 mass% and 0.004 mass% < Zr ≤ 0.100 mass%. It is one element. According to claim 1 or 2,
Steel with a mass of 3000 kg or more and the minimum of the longitudinal dimension (L ₁ ), transverse dimension (L ₂ ), and height dimension (L ₃ ) (L _min ) is 300 mm or more. Molds manufactured from steel according to claim 1 and having a mass of 2000 kg or more.